Hot runner injection nozzles are commonly used to dispense liquefied plastic into molds. It is essential that hot runner injection nozzles maintain a flowable, liquefied plastic material throughout the length of the nozzle and into the mold in order to properly dispense the liquefied plastic into the mold. This is accomplished by maintaining the plastic material at a proper elevated temperature. Maintaining the plastic material at the proper elevated temperature has proven to be burdensome in the prior art because the plastic material is subject to cooling as it moves away from the plastic injection molding machine and the main heating source, and into the nozzle.
Typical hot runner nozzles include a material tube through which the plastic material flows from the plastic injection molding machine to the mold and a valve or tip integral with or connected to an end of the material tube from which the material is dispensed into the mold. Many currently available nozzles having this configuration do not properly dispense liquid plastic material because the material cools substantially (cold material commonly referred to as cold slug) and becomes less flowable as the plastic material moves away from the main heating source, through the material tube and out the valve or tip. Cooling of the liquefied plastic causes it to leave deposits within the material tube and the valve or tip. Such deposits can solidify and hamper material flowability through the tube and the valve or tip. Sometimes material flow through the tube and the valve or tip can be completely blocked, in which case the nozzle may need to be replaced or cleaned. Flow of material through the valve or tip can also be hampered or blocked by foreign materials such as, for example, metal shavings entering the material tube or by material that burns and hardens within the material tube.
Some hot runner nozzles utilize a secondary heater positioned in the nozzle to maintain a proper elevated temperature of the plastic material. Such secondary heaters have been placed in contact with the material tube, thereby heating the material tube and the plastic material therein. Secondary heaters commonly terminate short of the valve or tip because of structural limitations of the nozzle. Since the secondary heaters do not heat the plastic material within the valve or tip, the plastic material loses heat in that area. Heat loss most commonly occurs by conduction through the walls of the material tube, the valve and/or the tip.
In instances where build-ups occur in the nozzles or the nozzle becomes completely blocked, the molding process may need to be interrupted to clean or replace the nozzle. Nozzles are typically burdensome to disassemble and to clean. Oftentimes, the nozzle simply will be replaced to avoid such burdensome work and the associated downtime. Repeated unnecessary replacement of hot runner nozzles adds unnecessary expense to the overall molding process. In addition, continuous purchasing and replacing of hot runner nozzles can be costly.